WO 97/45396 discloses N-bis-and N-tris-[(1,2-dicarboxy-ethoxy)-ethyl]-amine derivatives including N-bis-[(1,2-dicarboxy-ethoxy)-ethyl]-aspartic acid (also called aspartic acid diethoxy succinate or AES), and the use of these derivatives as chelating agents of metals, especially in connection with pulp bleaching. These derivatives can be prepared by reacting a di- or triethanolamine with an alkali metal or alkaline earth metal salt of maleic acid in the presence of a catalyst such as lanthanoid compounds, a nickel compound or alkaline earth metal compounds, e.g. calcium hydroxide or magnesium hydroxide.
A drawback with the above synthesis of e.g. AES is that the reaction is relatively slow, the reaction time being about 12 to 16 hours, and that the reaction does not go to completion. A typical obtainable conversion from diethanol amine to AES is about 60 to 70%. A significant amount of the rather expensive maleic acid, typically up to about 40%, is left unreacted.
The aim of the present invention is to improve the process for the preparation of AES and to make the process for the preparation of AES more profitable. Another aim of the present invention is to provide a method for in situ preparation of mixtures of complexing agents.